This invention relates to a loudspeaker diaphragm and a method of manufacture thereof. The term loudspeaker diaphragm includes the conventional round or elliptical generally cone-shaped structure, and also substantially planar membranes. The diaphragm also may have a perimeter other than circular or elliptical.
The design of a loudspeaker diaphragm, and the choice of material from which it is made, aims to provide a consistent response to an audio-frequency electric signal input to the voice coil (or other electroacoustic transducer) of the loudspeaker across its entire working frequency range.
This requirement brings into conflict the need for the cone to be light in weight, so that it can be driven efficiently by a relatively small signal, yet stiff enough to respond to the signal without unacceptable distortion or coloration of the sound. Paper cones whilst light in weight tend not to be stiff enough; cones of plastics materials tend to be relatively thick and heavy due to the limitations of the manufacturing process.
The present invention at least in its preferred embodiments seeks to provide a loudspeaker diaphragm which is less subject to the foregoing disadvantages.
The invention provides in one aspect a method of manufacturing a loudspeaker diaphragm comprising moulding it from a flowable plastics material such that part of the material flows during moulding a distance equal to at least 300 times the thickness of that part of material after moulding, the moulded material having a Youngs Modulus of at least 1500 MPa, and preferably at least 2000 MPa.
Preferably the said part of the material is injected into a mould having a moveable wall defining a mould gap, said wall being moved to reduce the gap and cause the said part of the material to flow said distance.
In another aspect the invention provides a loudspeaker diaphragm moulded from flowable plastics material having a Youngs Modulus of at least 1500 MPa (preferably at least 2000 MPa), a part of the cone having a flow length/thickness ratio (as herein defined) of at least 300.
The plastics material may be for example polypropylene or another polyolefin, or a polystyrene.
Preferably the flow length/thickness ratio exceeds 350, 400 or 500 or 600.
Preferably the thickness of the said part of the material does not exceed 0.20 mm. The material may contain reinforcing fibres, which may comprise one or more of carbon fibres, glass fibres, polyester fibres, vegetable fibres (eg. hemp or linen) or animal fibres eg. horse hair.
Preferably the Young""s modulus exceeds 2000, 2500 or 3000 MPa when the material is not fibre-reinforce or 5000 or 6000 MPa if it contains reinforcing fibres.
When the fibres are carbon fibres they may constitute from 25% to 50% by weight of the material.
The longitudinal extent of the fibres may exhibit a preferred orientation radially from a central region of the diaphragm.
This orientation can be produced when the flow length is generally radially of the diaphragm.
Different regions of the diaphragm may have different thicknesses, whereby to modify the acoustic properties of the diaphragm.
At least one said region may comprise an elongate rib; for example the rib may extend around a polar axis of the diaphragm.
Alternatively or in addition a said rib may extend radially of the diaphragm.
In another aspect the invention provides an injection moulded loudspeaker diaphragm having at least one elongate rib integrally moulded thereon.
In a further aspect the invention provides a loudspeaker diaphragm having a continuous elongate rib, or a generally arcuate rib extending around a polar axis of the diaphragm. The rib may be axisymmetric, if the diaphragm is of circular section eg. conical.
The invention also includes a loudspeaker having a diaphragm as set forth above, or made by a method as set forth above.